1. Field of the Invention
The present invention is directed to hollow-fiber membrane permeators which permit counter-current flow within the permeators as between the exterior or shell-side and the interior or tube side of each hollow fiber in the permeator. Counter-current flow is much more efficient than co-current flow or transverse or cross-flow as now employed in hollow fiber permeators.
2. Description of Related Art
In fabricating hollow-fiber permeators, the fluid contacting efficiency and the flow-geometry are very important to the overall effectiveness of the permeator package. This is particularly important in perstraction, membrane enhanced extraction and membrane reactor cases where efficient contacting of two fluids can result in significantly improved performance. In cross-flow operation the flow along the shell sides of each hollow fiber is perpendicular to the tube-side-flow. In co-current operation the flow on both the shell side and tube side of each hollow fiber is parallel but in the same direction.
In many cases, co-current or cross-flow operations result in severe losses in performance. In concentration driven processes such as perstraction, a sweep liquid is used to carry away a product that permeates through the membrane. Due to the necessity of maintaining a concentration gradient co-current flow is especially inefficient requiring an extremely high sweep flow rate to maintain an effective gradient.
Cross-current or transverse-flow is somewhat better, requiring less sweep material to maintain an effective gradient. Due to sweep recovery costs the amount of sweep needed to carry out an efficient separation is critical to the overall process economics.
Numerous permeator designs have been put forward that give efficient contacting between the fluids passing on the shell-side and tube-side respectively of the hollow fibers but these designs typically result in cross-flow/transverse-flow where the shell side fluid flows in a path that is perpendicular to the path of the tube side fluid, or in less than optimal counter-current flow patterns.
U.S. Pat. No. 4,923,679 teaches a hollow fiber membrane type oxygenator and a method for its manufacture. This device permits passage of blood through the tube-side of the hollow fiber while oxygen passes counter-currently along the shell side of the fibers. The module contains no internals to allow for efficient flow distribution down the length of the exterior of the hollow fibers. If instead of oxygen a liquid were passed along the shell side of the hollow fibers the inability to insure the uniformity of flow along all the fiber lengths would evidence itself in permeate polarization in those areas where shell-side liquid flow is inadequate. Such permeate polarization in a concentration gradient driven permeation process is highly detrimental to the overall process efficiency; it reduces the effective transfer surface area of the module.
U.S. Pat. No. 4,231,879 teaches an apparatus for the selective separation of matter through semi-permeable membranes. The membranes are in the form of hollow fibers. The apparatus comprises a plurality of elongated frame assemblies each defining an elongated contacting chamber packed with hollow fibers. The elongated chambers are stacked side by side in fluid communications. The interior and exterior of the hollow fibers are isolated so that there is no fluid communicator between the interior and exterior of the fibers except through the fiber walls. A first fluid is passed through the tube-side of the fibers while a second fluid is passed counter-currently along the shell side of the fibers. Shell-side flow is in cascade fashion passing through the first elongated chamber in counter-current flow then through a non-contacting flow reversal chamber to be redirected into a second elongated chamber, again in counter-current flow and then in like fashion through a series of non-contacting flow reversal chambers and elongated contacting chambers until the shell side fluid exits the last contacting chamber and leaves the apparatus. The concentration of any permeate in the final elongated chambers could match the concentration of the permeate accumulated by the shell side fluid as it passes through the preceding elongated chambers. Thus, in concentration gradient driven systems the efficiency of the final chambers as could be quite lower again, in effect, reducing the effective transfer surface area of the apparatus.
GB 1,500,945 teaches a dialysis apparatus comprising a series of elongated chambers filled with hollow fibers. The interior and exterior of the fiber are fluidly isolated from each other. Means are provided for permitting a serpentine flow of fluid along the shell side of the fiber. Consequently, then, shell side flow will be counter-current along the fibers in one chamber but co-current along the fibers through the next chamber in series, and alternating thereafter until the fluid finally exiles the apparatus. Alternately the shell side fluid can pass in zig-zag fashion perpendicular across the membranes in each chamber bundle (transverse flow) before exiting the apparatus.
U.S. Pat. No. 3,957,648 teaches a hollow fiber element wherein a leak proof band is wound in the form of a spiral the turns of which spiral are cylinders radially spaced from one another within the element jacket and hollow fibers extend axially through the jacket occupying the radially spaced cylinders of the spiral. Flow along the shell side of the hollow fibers is transverse or cross-flow across the fibers following the path of the spiral from the interior of the spiral to the end of the spiral in the element.
U.S. Pat. No. 4,220,535 teaches a multi-zoned hollow fiber permeator. The flow in this permeator is transverse to the orientation of the hollow fibers. The improvement in the design is the placing of at least one transverse oriented impermeable barrier intermediate along the length of the hollow fibers within the housing. These barriers are an attempt to reduce flow channeling in the hollow fiber bundle and thus reduce permeate polarization.
U.S. Pat. No. 4,906,362 teaches a membrane filter which comprises one or several tubular flow channels for liquid which is to be treated in the filter. The walls of the flow channels consist on the inner side of membranes, which are supported by a supporting structure. An outer casing surrounds the flow channels and limits a collection space for permeate between the casing and the flow channels. According to the invention the flow channels and the said collection space are connected to separate outer circuits, comprising pumps to bring the liquid which is to be filtered as well as permeate to circulate concurrently past the membrane filter. The collection space is filled by filling bodies, which constituent an essential mechanical hindrance for the flow of circulating permeate. Means in the form of disks provided with holes, which abut against the outer casing, are arranged at the inlet to the flow channels. These means are arranged to retain the filling bodies in the collection space and distribute the flow of circulating permeate over the same.
U.S. Pat. No. 4,929,259 teaches a hollow fiber membrane fluid separation module for tube side feed. In one embodiment provision is made for the use of sweep fluid to pass counter-currently along the shell side of the hollow fibers. In that embodiment the hollow fibers are bundled around a hollow core. The hollow fibers and hollow core extend through tube sheets at the ends of the bundles. The hollow core has fluid inlet means at one end and is plugged at the other. At the plugged end the hollow core has perforations to permit fluid exit into one end of the hollow fiber bundle. A series of circular baffles co-axial with the hollow core divide the hollow fiber bundle into a series of circular chambers. The baffles are spaced away from the tube sheets at either end of the bundle. Tube-side and shell-side flow are counter-current. The sweep liquid exiting the center core passes across the top of the baffles surrounding the hollow fibers, then passed down the circular chambers defined by the circular baffles, passing along the shell side of the hollow fibers in the circular chambers. The sweep fluid exits the circular baffle chambers and exits the vessel housing the hollow fibers. The baffles channel the sweep fluid flow counter-current by along the shell side of the hollow fibers as compared to cross or transverse flow.
While improved counter-current and cross-current flow hollow fiber elements have been described, it would be an improvement if, with regard to counter-current flow hollow fiber elements the counter-current flow past each fiber could be controlled so as to be uniform and substantially the same past each fiber, thereby insuring no permeate polarization and maximizing element efficiency.